JP4550867B2 - TCP session switching apparatus and method, program, and recording medium - Google Patents

Description

  The present invention relates to a technique for relieving a TCP (Transmission Control Protocol) session when a system is switched from an active system to a standby system in a duplex configuration of an active system and a standby system.

  A system that provides a network service is required to have extremely high reliability, that is, a capability of continuously operating over a long period of time (this is called availability). More specifically, the network service is provided 24 hours a day, 365 days a year, and typically the annual service availability is required to be 99.999% or more.

  For this reason, conventionally, a dedicated system has been used and availability has been secured by duplicating the system. However, the dedicated system is expensive, and there is a problem that the service cannot be provided at a low cost.

  Therefore, systems using general-purpose hardware and general-purpose OSs such as x86 architecture hardware and Linux (registered trademark) which is an open source OS have come to be used.

  For this reason, it is a problem to ensure the above-described availability in a system using general-purpose hardware or a general-purpose OS.

  Therefore, even in a system using Linux (hereinafter referred to as a Linux system), the server is configured in a cluster so that the active system and the standby system are duplicated. There is a need for technology to continue operation in the standby system. Here, the TCP session refers to the period from the establishment of the TCP connection to the disconnection. When a failure occurs, the replacement server taking over processing and data is referred to as failover. A technique for continuing operation from the active system to the standby system without being aware of switching is called a TCP session failover technique.

  In TCP / IP communication, one IP address is composed of a plurality (usually 65,536) “ports”. For this reason, when making a connection, it is necessary to designate a pair of an IP address and a port number. This set is called a socket.

  Therefore, in order to switch the service from the active system to the standby system, a mechanism is required that takes out the socket information of the active system from the Linux kernel and takes over to the standby system.

  As a technique for solving this problem, as shown in Non-Patent Document 1, a TCP Connection Passing (hereinafter referred to as TCPCP) technique has been reported.

The active application copies the status of the kernel, passes the copy to the standby application, and the standby application sets the status of the active kernel. To do.
W. Almerberger, "TCP Connection Passing", Proceedings of the Linux Symposium, p.9-22,2004

  In order to continue to the standby system without being aware of the switching of the TCP session, it is necessary to take over not only the socket information but also the packets accumulated in the buffer of the TCP part and the IP part at the same time. However, the packet may be transmitted even after acquiring the packet and socket information stored in the buffer. In that case, there is a possibility that a contradiction may occur between the sequence number and the time stamp of the packet already sent and the packet sent from the buffer after switching. When such a contradiction occurs, normal communication cannot be performed.

  As described above, TCPCP indicates a basic technique for taking over a socket from an active terminal to a standby terminal, and points out that a sequence number is an important parameter for ensuring synchronization of TCP connection. Further, it has been suggested that when a contradiction occurs due to a time stamp skipping in the past, it can be solved by discarding the conflicting packet or providing an offset. However, there has been a problem that practical technology has not been established.

  The present invention has been made in view of the problems of the prior art as described above, and smoothly switches from the active system to the standby system, and resolves the contradiction between the sequence number of the packet and the time stamp. The purpose is to provide technology.

A TCP session switching device according to the present invention includes a control unit, a socket unit, a TCP unit, an IP unit, a service server currently in operation and a plurality of IP addresses for service and control, and a standby And both the active and standby servers are connected to the opposite terminal via a switch, the active and standby systems share the same service IP address, and only in the active system A TCP session switching device that enables an IP address for general services,
The control unit of the active server obtains socket information from the socket unit, the TCP unit, and the IP unit of the active server, and stops the packet transmission / reception of the TCP session to be taken over by the active server at the time of system switching. , Stop the IP address to be taken over for the service, send a request signal indicating the request for system switching to the standby server and the acquired socket information to the standby server,
When the control unit of the standby server receives the request signal and socket information, it sets the received socket information in the socket unit, TCP unit, and IP unit of the standby server, and starts packet transmission / reception by the standby server,
The socket information includes a first sequence number indicating a sequence number indicating a next transmission number of the packet, a sequence number receiving a signal transmitted from the transmission destination computer to the transmission source computer, and a sequence number receiving next. 3 sequence numbers,
When the control unit of the active system and the standby system server receives the notification of the first and third sequence numbers from the opposing terminal after transmitting the packet to the opposing terminal, the first and third of the packets transmitted immediately before are received. The sequence number is overwritten on the notified sequence number and transmitted to the opposite terminal.

In this case, the socket information includes the packet timestamp,
When the control unit of the active system and the standby system server receives the time stamp notification from the opposite terminal after transmitting the packet to the opposite terminal, the time stamp of the packet transmitted immediately before is referred to the notified time stamp as a reference. It may be corrected and transmitted to the opposite terminal.

A TCP session switching system according to the present invention is a TCP session switching device including a currently operating active server and a standby server each including a control unit, a socket unit, a TCP unit, and an IP unit. A TCP session switching system comprising: an opposite terminal connected via a switch to either the active system or the standby server of the TCP session switching device,
The control unit of the active server includes a sequence number indicating the next transmission number of the packet from the socket unit, the TCP unit, and the IP unit of the active server, and a signal transmitted from the transmission destination computer to the transmission source computer. Socket information including the first to third sequence numbers indicating the sequence number received next and the sequence number to be received next is acquired, and during system switching, packet transmission / reception by the active server is stopped, and the standby system is Send a request signal indicating a request for system switching to the server and the acquired socket information to the standby server,
When the control unit of the standby server receives the request signal and socket information, it sets the received socket information in the socket unit, TCP unit, and IP unit of the standby server, and starts packet transmission / reception by the standby server,
The opposite terminal compares the first and third sequence numbers of the packets sent from the active or standby server with those of the previously received packet, and determines that it is a past one. Transmitting the first and third sequence numbers based on the sequence numbers of the previously received packets to the active or standby server,
When the control unit of the active or standby server receives the notification of the first and third sequence numbers from the opposite terminal after transmitting the packet to the opposite terminal, the first and third of the packets transmitted immediately before are received. The sequence number is overwritten on the notified sequence number and transmitted to the opposite terminal.

A TCP session switching system according to another aspect of the present invention includes a currently operating active server and a standby server each including a control unit, a socket unit, a TCP unit, and an IP unit. A TCP session switching system comprising: a TCP session switching device; and an opposite terminal connected via a switch to any of the active and standby servers of the TCP session switching device,
The control unit of the active server acquires socket information including a time stamp of the packet from the socket unit, the TCP unit, and the IP unit of the active server, and transmits and receives packets by the active server at the time of system switching. And sends a request signal indicating the request for system switching to the standby server and the acquired socket information to the standby server,
When the control unit of the standby server receives the request signal and socket information, it sets the received socket information in the socket unit, TCP unit, and IP unit of the standby server, and starts packet transmission / reception by the standby server,
The opposite terminal compares the time stamp of the packet sent from the active or standby server with that of the packet received last time, and if it is determined that it is a past time, Send the stamp to the active or standby server,
When the control unit of the active system or standby system server receives a time stamp notification from the opposite terminal after transmitting the packet to the opposite terminal, the time stamp of the packet transmitted immediately before is used as a reference for the notified time stamp. It correct | amends and it transmits to the said opposing terminal, It is characterized by the above-mentioned.

The program according to the present invention causes the above-described TCP session switching device to be realized in a computer system.

  The recording medium according to the present invention stores the above program.

  In the present invention configured as described above, when switching from the active server to the standby server, packet transmission / reception by the active server is stopped, and socket information is transferred from the active server to the standby server. Since packet transmission / reception by the standby server is started after the copying is completed, switching from the active server to the standby server can be performed smoothly.

  In addition to the socket information, if the sequence number and time stamp are corrected, the sequence number and time stamp are consistent, so the system can be switched from the active system to the standby system without being aware of system switching. There is an effect that can.

  Next, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram showing the configuration of a duplex system according to the present invention. The active server 101 and the standby server 201 are duplicated. The opposite terminal 301 is a terminal that is connected to the server 101 or 201 via the switch 302 and transmits / receives a packet to / from the server 101 or 201.

  The active server 101 and the standby server 201 are Linux systems.

  The kernel 102/202 includes a socket unit 103/203, a TCP unit 104/204, and an IP unit 105/205 for TCP / IP communication. The TCP unit 104/204 and the IP unit 105/205 include a transmission buffer and a reception buffer, and hold transmission packets and reception packets.

  Note that each of the active server 101, the standby server 201, and the opposite terminal 301 includes an input device such as a keyboard, a mouse, and a receiving device, a control device such as a CPU, a ROM, a RAM, an HDD, and the like. It is configured by a general computer system including a storage device that performs storage and the like and an output device such as a display, a printer, and a transmission device. Each control device performs processing according to programs stored in the ROM, RAM, and HDD, and is configured in the above-described kernels 102/202 in the active server 101 and the standby server 201 and in these. The socket unit 103/203, the TCP unit 104/204, and the IP unit 105/205 are virtually constructed in the computer system.

  In addition, programs stored in ROM, RAM, and HDD can be imported from a readable recording medium such as CD-ROM, DVD-ROM, floppy disk, or USB memory via these input devices, the Internet, etc. There is a form that is imported from the receiving device via the network. The system according to the present invention is constructed on a general computer system by these programs, and the present invention includes these programs and a recording medium storing the programs.

  FIG. 2 is a flowchart when switching from the active system to the standby system.

  In the standby server 201, packet transmission / reception is stopped by the socket stop function of the control unit 206, and packet filtering is applied.

  The socket stop function is a function for stopping packet transmission / reception, and operates when a socket stop request is received from the control units 106 and 206.

  Packet filtering is a function for stopping transmission of packets in the IP unit. If the standby server 201 receives a packet for an unset socket while setting socket information by the socket information setting function, the TCP session is disconnected. Therefore, the packet filtering of the IP unit 105 is stopped until the socket information setting process of the standby server 201 is completed by packet filtering.

  In step S401, considering the case of failover, the control unit 106 of the active server 101 acquires socket information from the socket unit 103, the TCP unit 104, and the IP unit 105 at an arbitrary timing ((( 1)).

  The socket information acquisition function is a function for passing socket information in the kernel 102/202 necessary for TCP session relief to the control unit 106/206, and operates when a socket information acquisition request is received from the control unit 106/206. .

  In step S402, the control unit 106 of the active server 101 generates a system switching trigger ((2) in FIG. 1).

  In step S403, in order to prevent a mismatch between the state of the control unit 106 of the active server 101 and the kernel 102 and to prevent a state mismatch with the opposite terminal 301, the control unit of the active server 101 The packet transmission / reception in the TCP unit 104 is stopped using the socket stop function 106 ((3) in FIG. 1).

  In step S404, the kernel 102 copies information necessary for recovering the TCP session and the contents of the transmission buffer and the reception buffer to the socket information.

  In step S405, the control unit 106 of the active server 101 acquires socket information from the kernel 102 using the socket information acquisition function ((4) in FIG. 1).

  In step S406, the control unit 106 of the active server 101 deletes the IP address after performing packet filtering so as not to transmit / receive a packet for IP address switching (FIG. 1 (5)).

  In step S <b> 407, the control unit 106 of the active server 101 transmits socket information to the control unit 206 of the standby server 201. Further, a system switching request is transmitted to the control unit 206 of the standby server 201 (FIG. 1 (6)).

  In step S408, the control unit 206 of the standby server 201 sets an IP address in the IP unit 205 ((7) in FIG. 1).

  In step S409, socket information is set in the kernel 202 by the socket information setting function of the control unit 206 of the standby server 201 ((8) in FIG. 1).

  The socket information setting function is a function for setting designated socket information for the TCP unit 104/204.

  In step S410, the control unit 206 of the standby server 201 starts packet transmission / reception using the socket start function ((9) in FIG. 1). If there is a packet to be sent to the send buffer, send it. Here, since there is a possibility that the sequence number and the time stamp of the standby server 201 are different from those of the opposite terminal, correction processing is performed.

  The socket start function is a function for restarting packet transmission / reception, and operates when a socket start request is received from the control unit 206.

  In step S411, the control unit 206 of the standby server 201 cancels packet filtering. In effect, communication with the opposite terminal 301 is resumed here ((10) in FIG. 1).

  In step S412, when it is confirmed that the standby server 201 resumes communication, the active control unit 106 closes the socket using the socket deletion function ((11) in FIG. 1).

  The socket deletion function is a function for deleting a socket of the active server 101 that is no longer needed, and operates when a socket deletion request is received from the control unit 106.

  As described above, since the socket information is directly transferred from the active server 101 to the standby server 201, there is an effect that the opposite terminal 3-1 is not aware of switching.

  In the above description, the control unit 106 of the active server 101 issues a trigger for switching. However, in the case of a failover, the control unit 106 of the active server 101 detects an abnormality and switches over (the master role is moved to the slave). ), The control unit 106 of the active server 101 may issue a trigger by a switching notification from the administrator to the control unit 106.

  FIG. 3 is a diagram showing a configuration and an operation when an error occurs during the switching process and it is necessary to switch back, and FIG. 4 is a flowchart showing the operation in detail. The configuration shown in FIG. 3 is basically the same as FIG.

  In step S413, the control unit 206 of the standby server 201 deletes the IP address. Packet filtering remains set ((12) in FIG. 3).

  In step S414, the control unit 206 of the standby server 201 transmits a system switch request again to the active server 101 ((13) in FIG. 3).

  In step S415, the control unit 106 of the active server 101 sets an IP address ((14) in FIG. 3).

  In step S416, the control unit 106 of the active server 101 starts packet transmission / reception by the socket start function ((15) in FIG. 3).

  In step S417, the control unit 106 of the active server 101 cancels the packet filtering ((16) in FIG. 3).

  In step S418, communication with the opposite terminal 301 is resumed.

  As described above, in this embodiment, even if an error occurs in the switching process, it is possible to immediately switch back, and there is an effect that the opposite terminal 301 is not aware of the switching.

  Next, the sequence number and time stamp correction processing will be described.

  The sequence number is uniquely set for each packet, and this value is updated for each transmission during session establishment. By adding the number of bytes transmitted each time to the sequence number, it is possible to uniquely determine how far the data stream has been transmitted / received.

  The timestamp is used to find out if there is an old packet among the packets. If the time stamp appears to be back in time, the opposite terminal 301 discards the packet.

  Even after the socket information is acquired by the active server 101, the active server 101 and the opposite terminal 301 may communicate with each other. Then, there is a possibility that the packet transmitted after the system switching is earlier in sequence number and time stamp than the packet already transmitted to the opposite terminal. If the sequence number and time stamp are inconsistent, normal transmission and reception cannot be performed. Therefore, the sequence number and time stamp are corrected only immediately after switching.

  FIG. 5 is a time chart showing the sequence number correction operation.

  Assume that packet transmission / reception is performed and socket information is acquired in the active server 101 (time 501). At that time, as the socket information, the sequence number that the active server 101 transmits next is seq1, and the sequence number that received ACK (positive signal sent from the transmission destination computer to the transmission source computer) is seq2. Assume that the next acknowledgment response number (ackseq) received is seq103.

  However, after the socket information is acquired, before the system is switched, packets are transmitted and received between the active server 101 and the opposite terminal 301. In the opposite terminal 301, the sequence number to be transmitted next is seq104, Assume that the sequence number that received the ACK is seq 105 and the confirmation response number is seq 6 (time point 502).

  Thereafter, when the system is switched from the active server 101 to the standby server 201, socket information is set in the standby server 201 (time 503), and packet transmission / reception is started by the socket start function (time 504). .

  The sequence number of the packet transmitted from the standby server 201 to the opposite terminal 301 is seq1 which is the initial state, and the confirmation response number (ackseq) is seq3. The opposite terminal 301 determines that the sequence number is different from the sequence numbers and seq 104 of the next response sequence number and the confirmation response number (time 505).

  Therefore, a D-SACK signal (defined in RFC 2833-An Extension to the Selective Acknowledgment (SACK) Option for TCP) is transmitted from the opposite terminal 301, the sequence number to be transmitted next is seq 104, and then the reception is performed. The standby server 201 is notified that the expected sequence number is seq6. Based on this, the sequence number to be transmitted next by the standby server 201 is seq6, the sequence number that received the ACK is seq6, and the confirmation response number (ackseq) that is received next is corrected by overwriting seq104 (time point 506). , Sequence number conflicts can be resolved.

  FIG. 6 is a time chart for time stamp correction.

  A time point 501 when the active server 101 acquires socket information from the opposite terminal 301 is defined as tsA1. After that, it is assumed that packets are transmitted and received between the active server 101 and the opposite terminal 301, and the active server 101 has received the packet until the time 502 of tsA2 after tsA1.

  Thereafter, switching from the active server 101 to the standby server 201 is performed. Thereafter, the socket information is set at time 503, and the socket is started and the packet is transmitted at time 504. The time stamp of the packet sent at this time is tsA1. Since this time stamp tsA1 is older than tsA2, the opposite terminal 301 discards the packet (time 505).

  Thereafter, the opposite terminal 301 transmits the time stamp tsA2 of the latest packet that has been successfully received to the standby server 201, and the standby server 201 measures the time based on the time and corrects the packet time (time point). 506).

  As a result, time stamp inconsistencies can be resolved, and there is an effect of preventing packets from being discarded due to switching from the active server 101 to the standby server 201.

  The present invention can be used in a system that provides a network service that requires high availability.

It is a figure which shows the structure of the duplication system concerning this invention. It is a flowchart at the time of switching from an active system to a standby system. It is a figure which shows a structure when it becomes necessary to switch back. It is a flowchart at the time of switching back from a standby system to an active system. It is a figure of the time chart of correction | amendment of a sequence number. It is a time chart about time stamp correction.

Explanation of symbols

101 Active server 201 Standby server 102, 202 Kernel 103, 203 Socket unit 104, 204 TCP unit 105, 205 IP unit 106, 206 Control unit 301 Opposed terminal 302 Switch 501 to 506 Timing S401 to S412 From active system to standby system Steps for switching S413 to S418 Steps for switching back from the standby system to the active system

Claims (10)

A control unit, a socket unit, a TCP unit, an IP unit, an active server currently in operation each having a plurality of IP addresses for service and control, and a standby server, TCP in which both the active and standby servers are connected to the opposite terminal via a switch , the active and standby systems share the same service IP address, and the general service IP address is enabled only on the active system A session switching device,
The control unit of the active server obtains socket information from the socket unit, the TCP unit, and the IP unit of the active server, and stops the packet transmission / reception of the TCP session to be taken over by the active server at the time of system switching. , Stop the IP address to be taken over for the service, send a request signal indicating the request for system switching to the standby server and the acquired socket information to the standby server,
When the control unit of the standby server receives the request signal and socket information, it sets the received socket information in the socket unit, TCP unit, and IP unit of the standby server, and starts packet transmission / reception by the standby server,
The socket information includes a first sequence number indicating a sequence number indicating a next transmission number of the packet, a sequence number receiving a signal transmitted from the transmission destination computer to the transmission source computer, and a sequence number receiving next. 3 sequence numbers,
When the control unit of the active system and the standby system server receives the notification of the first and third sequence numbers from the opposing terminal after transmitting the packet to the opposing terminal, the first and third of the packets transmitted immediately before are received. A TCP session switching apparatus, wherein the sequence number is overwritten on the notified sequence number and transmitted to the opposite terminal. The TCP session switching device according to claim 1,
The socket information includes the time stamp of the TCP header packet,
The control unit of the active system and standby server, after transmitting a packet to the correspondent node, wherein the more the opposite terminal receiving a notification of the time stamp, the time stamp of the packet transmitted immediately before, based on the notified time stamp The TCP session switching apparatus, wherein the TCP session switching apparatus corrects the data as follows and transmits the corrected data to the opposite terminal. A TCP session switching device comprising a control server, a socket unit, a TCP unit, and an IP unit, each of which is currently operated and a standby server, and the operation of the TCP session switching device A TCP session switching system comprising an opposite terminal connected to either a server or a standby server via a switch,
The control unit of the active server includes a sequence number indicating the next transmission number of the packet from the socket unit, the TCP unit, and the IP unit of the active server, and a signal transmitted from the transmission destination computer to the transmission source computer. Socket information including the first to third sequence numbers indicating the sequence number received next and the sequence number to be received next is acquired, and during system switching, packet transmission / reception by the active server is stopped, and the standby system is Send a request signal indicating a request for system switching to the server and the acquired socket information to the standby server,
When the control unit of the standby server receives the request signal and socket information, it sets the received socket information in the socket unit, TCP unit, and IP unit of the standby server, and starts packet transmission / reception by the standby server,
The opposite terminal compares the first and third sequence numbers of the packets sent from the active or standby server with those of the previously received packet, and determines that it is a past one. Transmitting the first and third sequence numbers based on the sequence numbers of the previously received packets to the active or standby server,
When the control unit of the active or standby server receives the notification of the first and third sequence numbers from the opposite terminal after transmitting the packet to the opposite terminal, the first and third of the packets transmitted immediately before are received. A TCP session switching system, wherein the sequence number is overwritten on the notified sequence number and transmitted to the opposite terminal. A TCP session switching device comprising a control server, a socket unit, a TCP unit, and an IP unit, each of which is currently operated and a standby server, and the operation of the TCP session switching device A TCP session switching system comprising an opposite terminal connected to either a server or a standby server via a switch,
The control unit of the active server acquires socket information including a time stamp of the packet from the socket unit, the TCP unit, and the IP unit of the active server, and transmits and receives packets by the active server at the time of system switching. And sends a request signal indicating the request for system switching to the standby server and the acquired socket information to the standby server,
When the control unit of the standby server receives the request signal and socket information, it sets the received socket information in the socket unit, TCP unit, and IP unit of the standby server, and starts packet transmission / reception by the standby server,
The opposite terminal compares the time stamp of the packet sent from the active or standby server with that of the packet received last time, and if it is determined that it is a past time, Send the stamp to the active or standby server,
When the control unit of the active system or standby system server receives a time stamp notification from the opposite terminal after transmitting the packet to the opposite terminal, the time stamp of the packet transmitted immediately before is used as a reference for the notified time stamp. A TCP session switching system, wherein the TCP session is corrected and transmitted to the opposite terminal. A currently operating active server and a standby server each including a control unit, a socket unit, a TCP unit, and an IP unit, and either the active or standby server is switched A switching method performed by a TCP session switching device connected to the opposite terminal via
The control unit of the active server obtains socket information from the socket unit, the TCP unit, and the IP unit of the active server, and stops the packet transmission / reception by the active server and switches to the standby server when the system is switched. Transmitting a request signal indicating a request for system switching to the standby server and the acquired socket information;
When the control unit of the standby server receives the request signal and socket information, the received socket information is set in the socket unit, TCP unit, and IP unit of the standby server, and packet transmission / reception by the standby server is started. And having
The first to third socket information includes a sequence number indicating the next transmission number of the packet, a sequence number receiving a signal sent from the transmission destination computer to the transmission source computer, and a next reception sequence number. Sequence number of
After the control unit of the active system and the standby system server transmits the packet to the opposite terminal and receives the notification of the first and third sequence numbers from the opposite terminal, the first and third of the packets transmitted immediately before are received. A TCP session switching method, comprising: overwriting the notified sequence number with the notified sequence number and transmitting the sequence number to the opposite terminal. The TCP session switching method according to claim 5 ,
The socket information includes the time stamp of the TCP header packet,
The control unit of the active system and standby server, after transmitting a packet to the correspondent node, wherein the more the opposite terminal receiving a notification of the time stamp, based on the timestamp of the packet transmitted immediately before the notified time stamp And a step of transmitting to the opposite terminal after correcting as a TCP session switching method. A TCP session switching device comprising a control server, a socket unit, a TCP unit, and an IP unit, each of which is currently operated and a standby server, and the operation of the TCP session switching device A switching method that is performed in a TCP session switching system including an opposite terminal connected to either of a server and a standby server via a switch,
The control unit of the active server sends a sequence number indicating the next transmission number of the packet from the socket unit, the TCP unit, and the IP unit of the active server, and a signal sent from the transmission destination computer to the transmission source computer Socket information including the first to third sequence numbers indicating the sequence number received next and the sequence number to be received next is acquired, and during system switching, packet transmission / reception by the active server is stopped, and the standby system is Transmitting a request signal indicating a request for system switching to the server and the acquired socket information to the standby server;
When the control unit of the standby server receives the request signal and socket information, the received socket information is set in the socket unit, TCP unit, and IP unit of the standby server, and packet transmission / reception by the standby server is started. When,
When the opposite terminal compares the first and third sequence numbers of the packets sent from the active or standby server with those of the previously received packet and determines that it is a past one, Transmitting first and third sequence numbers based on each sequence number of the previously received packet to the active system or standby system server;
When the control unit of the active or standby server transmits a packet to the opposite terminal and then receives a notification of the first and third sequence numbers from the opposite terminal, the first and third of the packets transmitted immediately before And a step of overwriting the notified sequence number over the notified sequence number and transmitting it to the opposite terminal. A TCP session switching device comprising a control server, a socket unit, a TCP unit, and an IP unit, each of which is currently operated and a standby server, and the operation of the TCP session switching device A switching method that is performed in a TCP session switching system including an opposite terminal connected to either of a server and a standby server via a switch,
The control unit of the active server acquires the socket information including the time stamp of the packet from the socket unit, the TCP unit, and the IP unit of the active server, and at the time of system switching, packet transmission / reception by the active server Transmitting a request signal indicating a request for system switching to the standby server and the acquired socket information to the standby server;
When the control unit of the standby server receives the request signal and socket information, the received socket information is set in the socket unit, TCP unit, and IP unit of the standby server, and packet transmission / reception by the standby server is started. When,
When the opposite terminal compares the time stamp of the packet sent from the active or standby server with that of the packet received last time and determines that it is a past time, the time of the packet received last time Sending a stamp to the active or standby server;
When the control unit of the active or standby server receives a time stamp notification from the opposite terminal after transmitting the packet to the opposite terminal, the time stamp of the packet transmitted immediately before is used as a reference for the notified time stamp. And a step of correcting and transmitting to the opposite terminal. The program which makes a computer system implement | achieve the apparatus of Claim 1 or Claim 2. A recording medium storing the program according to claim 9 .

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